Biomedical Engineering Reference
In-Depth Information
strain KC secretes a compound (pyridine-2,6-bis-thiocarboxylate, or PDTC) that promotes
chemical dechlorination of CT outside the cell.
An alternative to bioaugmentation with strain KC is biostimulation, where sufficient
organic carbon is added to drive the system into sulfate-reducing or methanogenic conditions
(i.e., low pE environments). Under these conditions, CF produced from CT can be sequentially
reduced and dechlorinated to dichloromethane , which is susceptible to oxidation and hydrolysis
(Vogel et al., 1987 ). For high pE (i.e., oxidized) aquifers, however, use of such a process requires
addition of sufficient organic carbon to remove oxygen, nitrate, and ferric iron, with a potential
for accumulation of volatile fatty acids, hydrogen sulfide (H 2 S), ferrous iron, carbon disulfide,
ammonium, methane, and biomass in the resulting low pE environment. Strain KC bioaug-
mentation avoids these drawbacks: its growth occurs under high pE conditions (aerobic and
denitrifying) and even at relatively low cell concentrations, it can sustain adequate rates of CT
transformation. As a result, bioremediation can be accomplished with relatively low levels of
added organic carbon and without the unwanted byproducts typical of reduced environments.
The following sections detail the chemistry of CT transformation by the secreted agent
PDTC, its structure and reactivity, the genes required for its synthesis, and a cellular mecha-
nism proposed for its regeneration. The chapter then describes the transport and ecology of
strain KC for bioaugmentation applications, and concludes with a case study.
9.2 PHYSIOLOGICAL FUNCTION OF PDTC PRODUCTION
Early observations associated with the discovery of CT transformation by strain KC
determined that it is an iron-regulated process (Criddle et al., 1990 ). That original work
employed a medium in which metals were precipitated by autoclaving with calcium, magnesium
and phosphate salts. Adding iron increased the growth yield of the organism, but precluded CT
transformation. This finding led Criddle and coauthors to postulate that the physiological
function of the components responsible for CT transformation was to scavenge iron. Later
work identified the agent of CT transformation as PDTC (Lee et al., 1999 ), shown in Figure 9.1 .
PDTC had been discovered previously by natural products chemists investigating extracel-
lular products of pseudomonads excreted in response to iron stress (Hildebrand et al., 1983 ).
PDTC had thus been assumed to be a siderophore (iron-regulated, extracellular ferric iron
chelator with cognate receptor transport) since its discovery almost 30 years ago. An accurate
understanding of its function was important because efforts to promote its optimal production
typically required some manipulation of site conditions. Different sets of environmental signals
Figure 9.1. Structure of the PDTC.
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